diff --git a/index.js b/index.js
index 0e8ee03..2cdea49 100644
--- a/index.js
+++ b/index.js
@@ -459,6 +459,15 @@ module.exports = postcss.plugin('postcss-modules-local-by-default', function(
         return;
       }
 
+      if (
+        rule.nodes &&
+        rule.selector.slice(0, 2) === '--' &&
+        rule.selector.slice(-1) === ':'
+      ) {
+        // ignore custom property set
+        return;
+      }
+
       const context = localizeNode(rule, options.mode);
 
       context.options = options;
diff --git a/test.js b/test.js
index 53123cb..98baf64 100644
--- a/test.js
+++ b/test.js
@@ -512,6 +512,11 @@ const tests = [
     input: '.a { content: "\\2193 \\2193 \\2193" }',
     expected: ':local(.a) { content: "\\2193 \\2193 \\2193" }',
   },
+  {
+    should: 'not ignore custom property set',
+    input: ':root { --title-align: center; --sr-only: { position: absolute; } }',
+    expected: ':root { --title-align: center; --sr-only: { position: absolute; } }',
+  },
 ];
 
 function process(css, options) {